This invention relates to collision preventive devices which, being provided in medical equipment having moving parts, furnish protection against mechanical hazards that may arise from collisions of the moving parts with the patient or other parts of the equipment; more particularly, it relates to collision preventive devices adapted to be used in such type of medical equipment that comprises moving parts to which accessary parts may be attached.
Referring to FIG. 1 of the drawings, let us first describe, as an example of medical equipment having moving parts to which accessary parts may be attached, a therapeutic linear accelerator. To a fixed support 51 is mounted a rotatable gantry 52, which rotates around the rotational axis 52a. Further, a collimator head 53, mounted on the rotatable frame 52, rotates with respect thereto around the axis 53a, to limit the field 53b of radiation generated within and emitted from the linear accelerator. The rotation axis of the collimator head 53 constitutes the central axis of the flux of radiation 53b. The point of intersection 54 of the two rotational axes 52a and 53a constitutes a central point of therapy, called an isocenter. The patient 55 under radiation therapy lies on a table top 56, whose position is determined by means of a table 57. At the attachment location 58 for accessary parts at the bottom of the collimator head 53 may be attached an accessary, such as an electron beam applicator 59 utilized in the radation therapy, as illustrated in the figure; accessaries which may be attached to the attachment location 58 of the collimator head 53 instead of the applicator 59 include a shadow tray, hollow out mechanism, and a wedge filter. In the following, the applicator 59 is taken as an example of the accessary attached to the attachment location 58 of the collimator head 53.
In operation, the rotatable gantry 52 is rotated around its axis 52a according to the therapy program for the patient 54. Hence, the movement of the moving parts of the equipment and that of the accessary attached thereto may cause mechanical hazards. Thus, generally, medical equipment such as the above radiation therapy equipment having moving parts comprises contact detectors for detecting the contacts of the moving parts (or the accessary attached thereto) with the patient or other parts of the equipment, so that mechanical hazards arising from collisions of the moving parts or of the accessary attached thereto with the patient, etc., may be prevented. Thus, the medical equipment shown in FIG. 1 comprises contact detectors 60 and 61 provided on the collimator head 53 and the applicator 59, respectively. The detection and prevention of the collisions of the moving parts or the attached accessary may be effected by means of these contact detectors 60 and 61 with the help of a contact detection circuit of the collision preventive device shown in FIG. 2, which is disclosed in Japanese laid-open Pat. application No. 62-40182. The organization and method of operation of the collision preventive device of FIG. 2 is as follows.
FIGS. 2b and 2c show two alternative forms of the conventional contact detection circuit, whereby the switchs 4 and 6 operated by the contact detectors 60 and 61 (shown in detail in FIG. 2a) are coupled to the interlock circuit 24 for stopping the movement of the moving parts of the medical equipment, etc. In both figures, the reference numeral 4 represents switches operated by the contact detector 60 on the collimator head 53; the reference numeral 6 represents those operated by the contact detector 61 on the applicator 59.
In the case of the circuit of FIG. 2b, the switches 4 and 6 are normally closed and are opened, respectively, when the respective contact detectors 60 and 61 come into contact with an object. The serially connected switches 4 operated by the contact detector 60 and the serially connected switches 6 operated by the contact detector 61 are coupled to the interlock circuit 24 via independent serial circuits; thus, the switches 6 of the detector 61 are connected directly to the interlock circuit 24 via the connector 62. Hence, when the detectors 60 and 61 come into contact with an object to open at least one of the switches 4 and 6 the interlock circuit 24 activates a control sequence for stopping the movement of the moving parts of, or the generation of radiation within, the medical equipment.
In the case of the circuit of FIG. 2c, on the other hand, the switches 4 and 6 are normally open and are closed respectively when the contact detectors 60 and 61 come into contact with an object. The parallel switches 4 operated by the contact detector 60 and the parallel switches 6 operated by the contact detector 61 are coupled in parallel via a connector 62, so as to be coupled together, in parallel circuit relationship, to the interlock circuit 24. Thus, when the detector 60 or 61 comes into contact with an object, to close at least one of the switches 4 or 6, the interlock circuit 24 senses an occurance of such contact by the closure of the circuit and activates a sequence for stopping the movement of the moving parts of, or the generation of radiation within, the medical equipment.
The contact detection circuit shown in FIGS. 2b and 2c, however, have the following disadvantages. In the case of the contact detection circuit of FIG. 2b, the state in which the applicator 59 (an accessary) is not attached and the state in which an occurance of contact of the applicator 59 takes place are identical; hence, means for detecting the attachment of an accessary is indispensable. Further, two independent sensing means for detecting the opening of the switches 4 and 6 must be provided. As a result, the organization of the contact detection circuit becomes complicated. On the other hand, the contact detection circuit of FIG. 2c becomes incapable of contact detection, if there is an electrical disconnection along the circuit; this may present a grave danger if such electrical disconnection, which may occur at the connector 62 or along the wiring of the circuit, is not discovered. In addition, since the states in which the applicator 59 is properly attached and in which it is not attached cannot be distinguished, a further detecting means for detecting the proper attachment of accessaries is generally necessary or at least desirable. This accessary attachment detecting means must be implemented in the form of a separate circuit, and thus entails a substantial complication of the overall organization of the detection circuit.